The production of agricultural goods and in particular food and feed production, in sufficient quantity and quality is an increasingly challenging task. One the one hand, there is a continuous growth of the demand for agricultural products, due to increase in world population as well as increase in the average standard of living for large parts of the world population. On the other hand, the area suitable or available for agriculture is continuously shrinking, partly because of changing climate conditions which can result in deterioration of areas previously suitable for agriculture. A continuous demand exists to increase the yield potential of agricultural crops, or at least maintaining such yield potential when growing agricultural crops under suboptimal or adverse abiotic conditions.
Up to now, efforts to increase the intrinsic yield potential have mainly focused on exploiting the genetic variability within the crops. By traditional breeding techniques existing or induced variant alleles are shuffled into new combinations. More recently, the pool of variability has been expanded through molecular techniques allowing the exchange of genetic material across species, and even kingdom, barriers.
However, much less attention has been devoted to the role epigenetic control mechanisms may play in determining quantitative traits such as yield. Indeed, all quantitative traits such as size and weights in animals or yield, particularly seed yield in crops exhibit variability with a normal distribution, even within a population of genetically identical individuals. Underlying the observed phenotypic variability are genetic components, environmental factors but also epigenetic components. The importance in plants of epigenetic control components in short and long term adaptation to stress has been documented (Molinier et al. 2006, Transgeneration memory of stress in plants. Nature 442, 1046-1049). Furthermore, it has been demonstrated that altered epigenetic states can be transmitted to successive generations that have not been or are no longer exposed to the inducing trigger (also reviewed in Jablonka and Raz, 2009 Transgenerational epigenetic inheritance: prevalence, mechanisms, and implications for the study of heredity and evolution. The Quarterly Review of Biology 84, No. 2, 131-176).
There is no direct proof however that recurrent selection allows to drive the epigenetic component of a quantitative trait towards the ends of the normal distribution curve. In other words, the prior art remains silent on the ability to influence or select from a population, particularly from a genetically uniform population, those individuals with an above or below average value for the quantitative trait and fix that selected epigenetic component in a (sub)population.
Enfield and colleagues (Enfield, F. D., Comstock, R. E. & Braskerud, O, Selection for pupa weight in Tribolium castaneum. I. Parameters in base populations. Genetics 54, 523-533 (1966); Kaufman, P. K. & Enfield, F. D., Comstock, R. E. Stabilizing selection for pupa weight in Tribolium castaneum. Genetics 87, 324-341 (1977) described experiments whereby starting from a population obtained through a cross between two inbred flour beetles with an average pupa weight of 2400 μg, they were able to select a population with an average pupa weight of 5800 μg by continuous inbreeding for 120 generations. Enfield et al expressed their surprise that so many genetic variability was maintained over so many generations of inbreeding. It is however not unlikely that at a certain stage of the selection process epigenetic variability rather than genetic variability was selected for. This is further supported by identical experiments starting with inbred lines that were homozygous for the genes affecting pupa weight (Enfield, F. D. & Braskerud, O. Mutational variance for pupa weight in Tribolium castaneum. Theor. Appl. Genet. 77, 416-420 (1989)).
Various parameters have been employed to establish a correlation with the yield potential of a plant. A positive correlation has been found between yield potential and lower cellular respiration rates.
Wilson described the response to selection of dark respiration rate of mature leaves in Lolium perenne and its effects on growth of young plants and simulated swards. (Wilson Ann. Bot. 49, 303-312 (1982))
Wilson and Jones described the effect of selection for dark respiration rate of mature leaves on crop yields of Lolium perenne cv. S23. (Wilson and Jones Ann. Bot. 49, 313-320 (1982)).
Kraus et al. reported on the yield advantage of a ‘slow-’ over a ‘fast-’ respiring population of Lolium perenne cv. S23 which depends on plant density. (Kraus et al. New Phytol. 123, 39-44 (1993)) and on the effect of handling on the yield of two populations of Lolium perenne selected for differences in mature leaf respiration rate (Kraus et al. Physiol. Plant. 89, 341-346 (1993)).
Nunes-Nesi et al. described enhanced photosynthetic performance and growth as a consequence of decreasing mitochondrial malate dehydrogenase activity in transgenic tomato plants. (Nunes-Nesi et al. Plant Physiol. 137, 611-622 (2005)). Juczczuk et al. reported on the effect of mitochondrial genome rearrangement on respiratory activity, photosynthesis, photorespiration and energy status of MSC16 cucumber (Cucumis sativus) mutant. (Juczczuk et al, Physiol. Plant. 131, 527-541 (2007)).
De Block and De Brouwer described a simple and robust in vitro assay to quantify the vigour of oilseed rape lines and hybrids. (Plant Physiol. Biochem. 40, 845-852 (2002))
WO02/066972 provides methods and means for determining parent inbred plant lines with good combining ability, for determining good combinations of parent inbred plant lines capable of yielding hybrid lines with high heterosis, and further for determining the agronomical performance of different plant lines, which can be performed in vitro by determining the electron flow in the mitochondria under control and stress conditions
None of the prior art documents however described recurrent selection for low cellular respiration rates in a genetically uniform population of plants which allows to establish (sub)populations of plants which have a higher yield potential and tolerance to adverse biotic conditions. The epigenetic component can further be inherited in successive generations (and behaves as a dominant or co-dominant factor).